1. Field of the Invention
The invention relates to a magnetic resonance apparatus, comprising a magnet system for generating a steady magnetic field and a coil system for generating mutually perpendicular gradient fields.
2. Description of the Prior Art
A magnetic resonance apparatus of this kind is known from EP-A-216590.A gradient coil system described therein comprises a number of saddle coils for generating X- and Y-gradient fields for a magnetic resonance apparatus in which the direction of the steady magnetic field coincides with the (Z-) axis of a cylindrical measuring space. A gradient coil system of this kind is usually surrounded by one or more electrically conductive cylinders, for example walls and radiation shields of a superconducting magnet. When the current in gradient coils is switched, eddy currents are induced in these electrically conductive cylinders, which eddy currents counteract the desired gradient fields. The attenuation of the effective gradient field can partly be compensated for by means of an adjustable "overshoot" in the current through the gradient coils. The overshoot in the current (and hence also the electrical power to be delivered) is highly dependent on the ratio of the diameter of the smallest shield surrounding the coil and the diameter of the coil itself. The electrical power for the coil increases as the diameter of the electrically conductive cylinder decreases.
The eddy currents not only attenuate the desired gradients but also distort the gradient fields. The relationship between location and effective field strength is then different from the situation where the gradient coil is arranged so as to be isolated. Moreover, this relationship between field and location will generally be time-dependent, because the eddy currents will decay with a given time constant. Notably this time-dependency will generally lead to undesired distortions in the MR images.
If the eddy currents were not to decay in time, the effect of the eddy currents would remain limited to an increase of the electrical power required and the distortion of the images obtained. The distortion could in principle be avoided by taking the eddy currents into account in the design of the coil, so that the effective field would have the desired shape as well as possible. The decay of the eddy currents could in principle be prevented by surrounding the gradient coils by means of a superconducting shield. However, this will be difficult to realize for the time being. An alternative method, offering a comparable result, consists in the mounting of a system of "shielding coils" which are designed so that therein the current patterns are simulated which would arise in a superconducting shield. By powering this shielding coil with an active power supply, the time-dependency of the eddy current field will be eliminated.
Such a system of shielding coils, however, does not offer a solution to the electrical power required for the current overshoot; to the contrary, when the current in the coil is switched the effect of the shielding coil is substantially the same as the effect which would be exerted by a conductive shield on the diameter of the shielding coil and, because this diameter will always be smaller than the diameter of the cryostatic shields in which otherwise the eddy currents would be introduced, the total power required will be substantially higher.